The present invention relates to the centering of a disc-shaped record carrier on a turntable, which centering is effected relative to the radial direction, with associated alignment of the record carrier in the vertical, or axial, direction.
Disc-shaped record carriers are known which are provided with centering means in their center for positioning the record carrier in a centered manner during the playback process. In the simplest case the centering means are constituted by a hole in the center of the record carrier. A preferably conical centering shaft is introduced into this hole so as to center the record carrier in the radial direction. Vertical positioning, or alignment, is produced by contact with the turntable. For a video or audio record on which information is stored according to the high density storage technique, centering by means of a conical shaft is no longer sufficient. The record carrier must be centered radially with such precision that an eccentricity of less than 0.02 mm will not be exceeded. Such an extremely small range cannot be attained by a conical centering shaft which is fitted into the center of the record.
To nevertheless attain such extreme centering accuracy, record carriers are manufactured which are provided with centering means disposed at the same side as that on which the information is picked up. In this way, the centering is always matched to the side being played. A frustoconical raised portion can be provided for this purpose on each side of the record carrier and this raised portion is arranged precisely centrically with respect to the recording grooves on the same side. Achievement of a perfectly centric position of the centering means relative to the grooves would assure that during playback the playback stylus will always move only in one radial direction. The record carrier itself is held on the turntable during playback by a magnet which cooperates magnetically with a metal ring inserted into the record carrier. Vertical alignment with respect to the playback plane of the record carrier is produced by contact with the turntable.
Since two centering processes are required to center the record carrier, namely radial and vertical alignment, one centering process may easily interfere with the other. It is necessary for the record carrier to first to aligned radially, and the vertically on the turntable. This is the case as well when the record is guided by a frustoconical shaft, or hub, in a matching centering recess in the record carrier.
Due to the radial cnetering by means of a frustoconical raised portion or recess on the side of the record carrier bearing the information to be played back, it is necessary to perform radial centering on this side as well. Bringing the conical centering means through a hole is not sufficient. Vertical and radial centering must take place simultaneously. This would require a double fitting which could have the result of an unsatisfactory centering.